Recently, in accordance with the trend of smaller and lighter portable electronic equipment, the need for high performance and high capacity batteries used for the power sources for such equipment is increasing. At present, commercially available lithium rechargeable batteries are 4 V-grade batteries with an average discharge potential of 3.7 V. These are rapidly being employed for the so-called 3 C devices comprising cellular phones, notebook computers, and camcorders, which have become essential elements in the digital era.
Along with improving the capacity and performance characteristics of batteries, studies to improve the safety such as overcharge characteristics are being actively conducted. Upon overcharge of a battery, depending on the battery's state of recharge, an excess amount of lithium is deposited on the positive electrode, while an excess amount of lithium is inserted into on the negative electrode. Consequently, the positive and negative electrodes are thermally unstable which can result in a rapid exothermic reaction such as is due to the decomposition of the organic solvent, and can also lead to a thermal runaway phenomenon causing a serious safety problem.
To overcome such problems, aromatic compounds have been added to the electrolyte as redox shuttle additives. For example, U.S. Pat. No. 5,709,968 discloses a non-aqueous lithium ion battery that prevents the thermal runaway phenomenon resulting from overcharge by adding a benzene compound such as 2,4-difluoroanisole to the electrolyte. In addition, U.S. Pat. No. 5,879,834 discloses a method for improving battery safety by adding a small amount of an aromatic compound such as biphenyl, 3-chlorothiophene, furan, or the like to the electrolyte which is electrochemically polymerized during an unusual overvoltage condition to increase the internal resistance of the battery. Such redox shuttle additives increase the internal temperature of a battery at an early stage with the heat generated by the oxidation-reduction reaction shutting down the pores of a separator quickly and uniformly to prevent an overcharge reaction. Moreover, upon overcharge, the polymerization reaction of the redox shuttle additive on the surface of the electrodes consumes the overcharge current, further protecting the battery.
However, as batteries increase in capacity in accordance with customers' needs, the use of such additives cannot fully satisfy the requirement for a high level of battery safety. Consequently, with the increasing demand for high capacity batteries, alternative additives or electrolyte systems are required to ensure battery safety.
Furthermore, Japanese Patent Laid-Open Nos. 1997-22722 and 1996-306387 disclose a lithium ion battery with an improved affinity between the carbon electrode and the non-aqueous electrolyte, and with improved energy density, by using an electrolyte solution including a liquid- phase organic solvent selected from esters, ethers, and phenyl group-containing carbonates having a molecular weight of 108 to 220, in which the carbon electrode is used after dipping in the electrolyte solution. Nonetheless, such batteries have decreased safety upon high-rate overcharge as well as upon exposure to high temperature.